Under abuse conditions, high energy density electrochemical cells can leak or rupture which can cause damage to the device employing the cell. Examples of abuse conditions for a cell are abuse charging of the cell, forced discharging and external shorting. Such conditions cause the internal temperature of the cell to rise with a corresponding increase in pressure. Although such cells typically employ a venting mechanism wherein the electrolyte is expelled, the electrolyte can itself cause damage. Additionally, in cells which employ lithium as an anode material, if internal cell temperatures reach above 180.degree. C., the lithium can melt and result in a fire. Therefore, safety devices other than venting means which will disconnect the electrical circuit under abuse conditions are desirable. If the circuit can be disconnected, the rise in the cell's internal temperature and pressure as a result of the abuse conditions can be terminated.
Japanese patent application 85/249241 describes a hermetically sealed cell in which the metal cell container and cover are joined by a low melting metal alloy (solder) to form a temperature-sensitive venting means.
U.S. Pat. No. 3,855,066 discloses a nonaqueous lithium cell having an organic electrolyte and having an electrolyte expulsion vent in the cell container which vent opening is sealed with a heat fusible metal plug.
U.S. Pat. No. 4,397,919 discloses a thermal venting mechanism for a hermetically sealed non-cylindrical cell (e.g., a Li/SO.sub.2 cell) which employs a low-melting plug (e.g., in the electrolyte fill tube) made of a material such as a metal alloy which will melt below the explosive temperature of the cell. In addition, a thermoplastic sphere may be seated within the opening below the metal plug to form a pressure-tight seal against the opening walls. The cell container is made of a flexible material which can distort in response to internal pressure increase. When excessive internal or external heating occurs, the thermoplastic sphere deforms and partially releases the internal pressure. At higher temperatures (and after the pressure-induced volume expansion has occurred) the metal plug melts and provides a direct vent to the atmosphere.
U.S. Pat. No. 4,008,354 discloses a vent means employing an extrudable hot melt adhesive material which fills a vent hole and also covers part of the container surface surrounding the vent hole. Under abnormal internal pressure conditions, the adhesive material is extruded through the vent hole to provide a vent path for pressure release.
European Patent No. 125037 discloses a glass-to-metal (GTM) sealed cell having a pressure- and temperature-sensitive vent structure. This vent consists of a disc-shaped cap over a vent opening, a thin membrane covering the opening and a layer of fusible material between the membrane and the cap. The cap is provided with a pointed projection which punctures the flexible membrane after the fusible material has melted.
U.S. Pat. No. 4,855,195 discloses electrochemical cells that employ a current collector-safety switch member comprised of a shape memory alloy in the electric circuit in the cell. The collector-safety switch member has a base portion and a plurality of legs extended therefrom and when the internal temperature of the cell rises, the legs of the collector-safety switch member are retracted to thereby disconnect the electrical circuit in the cell. These cells can also be comprised of a resettable thermal switch for inhibiting the flow of current in the cell at a temperature below the disconnect temperature of the current collector-safety switch member.
In U.S. Pat. No. 4,975,341 an electrochemical cell is disclosed which employs a resettable or nonresettable safety disconnect device operable by a shape memory alloy element. The shape memory alloy element is preferably in the form of an accordion or coiled configuration that will be extended in normal operation of the cell and contract when the internal temperature exceeds a preselected value whereupon the shape memory alloy will break contact in the electrical circuit of the cell thereby rendering the cell inoperative.
It is an object of the present invention to provide a safety circuit disconnect device for use in electrochemical cells.
It an another object of the present invention to provide an externally mounted safety circuit disconnect device that can operate reliably to break the electronic contact with a terminal of the cell when the cell is subjected to abuse conditions which increase the temperature within the cell above a predetermined level.
It is another object of the present invention to provide a circuit disconnect device that employs a heat fusible conductive material that is easy to produce and cost effective to use in the cell.
The foregoing and additional objects will become fully apparent from the following description and the accompanying drawings.